Field-effect transistor

ABSTRACT

An organic FET  1  comprises a substrate  2  on which a gate insulation film  41  and a reformed layer  43  are formed in this order, and a source electrode  6  and a drain electrode  8  are further arranged thereon at a predetermined distance from each other, and furthermore, an organic semiconductor layer  10  is formed on and between the electrodes  6  and  8 . The reformed layer  43  fixed on the gate insulation film  41  and attached to the organic semiconductor layer  10  contains a specific compound containing the CN group or is composed of only a specific compound containing the CN group.

TECHNICAL FIELD

The present invention relates to a field-effect transistor (FET) and,more particularly, to an FET comprising semiconductor layers containingorganic substances.

BACKGROUND ART

Generally, in the case of a thin film organic FET using organicsemiconductors, organic semiconductor layers can be formed by a simpleprocess such as a printing method, a spray method, or an ink-jet method,therefore, the cost is considerably lower than that of an FET usinginorganic semiconductors. Moreover, since there is a possibility that alight and thin integrated circuit having a large area may be formedeasily, the application thereof to a liquid crystal display, an organicEL display, an IC card, etc., is expected.

Recently, the mobility of carrier of the organic semiconductor isincreased and those having the mobility of carrier as high as that ofthe amorphous silicon have been found. The research on how to put topractical use an FET using organic semiconductors having such a highmobility is extensively being carried out. Specifically, organicmaterials that exhibit a high mobility and are currently availableinclude pentacene, polyalkylthiophene, etc., as a result, a greatprogress in the development of the organic FETs has been found.

However, even though these materials are used, such a high mobility asthat of the amorphous silicon can be obtained only when the materialsare molecular substances and are used in the form of a single crystal.If the entire semiconductor layer is made up of a single crystal, it isextremely difficult to manufacture a large integrated circuit at a lowcost. On the other hand, organic semiconductors made of polycrystallineand amorphous polymers cannot be put to practical use because of themobility incommensurably lower than that of a single crystal due to, forexample, the loss of scattered electrons at grain boundaries. Therefore,in order to prevent a material from being brought into a polycrystallinestate, in other words, to prevent occurrence of crystal defects, aconsiderable amount of man-hours are paid for purification of thematerial to reduce the concentration of impurities in an organicsemiconductor layer as much as possible.

In order to solve these problems, a proposal is made in which themobility of a sexithiophene evaporated film, which is an organicsemiconductor layer, is increased by using cyanoethyl pullulan as amaterial for a gate insulation film in an FET structure (refer to Patentdocument 1).

Patent document 1: Japanese Patent No. 2984370

DISCLOSURE OF THE INVENTION

Problem to be Solved by the Invention

However, the inventors of the present invention examined in detail thecharacteristics of the conventional organic FET described above, inwhich cyanoethyl pullulan was used for a gate insulation film, and foundthe following problems. As described in Patent document 1 describedabove, if cyanoethyl pullulan is used for a gate insulation film, thedrain current can be increased but the responsiveness to theincrease/decrease in the drain current to the change of the gate voltageand the stability are insufficient. Specifically, according to theadditional test conducted by the inventors of the present invention, itwas found that after the application of the gate voltage, it took morethan tens of seconds until the drain current was stabilized. Moreover,an unstable behavior was observed, in which the drain current that hadgradually increased began to decrease as time elapsed.

When the responsiveness to the drain current to the change of the gatevoltage and the stability are insufficient, it is actually extremelydifficult to control the current flowing between the source electrodeand the drain electrode to a desired quantity using the gate voltage andit is substantially impossible to obtain desired transistorcharacteristics. In other words, in the case of the conventional organicFET in which cyanoethyl pullulan is used for a gate insulation film, theeffect of the increase in the drain current is marked and it may not bepossible to realize a practical organic FET without changing theconventional configuration.

The conventional organic FETs other than those described above have aproblem in that the current having flowed once decreases gradually astime elapses (for example, several seconds to several minutes), which isa characteristic inherent in an organic semiconductor layer, and becauseof the low mobility, it is extremely difficult to obtain a sufficientquantity of the drain current.

The above-mentioned problem being taken into consideration, the presentinvention has been developed and an object thereof is to provide an FETcapable of preventing the change of the drain current as time elapsesafter the application of the gate voltage and obtaining a stable draincurrent for a long time.

Means for Solving Problem

In order to attain the above-mentioned object, the inventors of thepresent invention focused on the physical properties of a gateinsulation film adjacent to an organic semiconductor layer and as aresult of an intensive study, it was found that a drain current-gatevoltage characteristic different from the conventional one could beobtained by providing a specific substance at the boundary surfacebetween the organic semiconductor layer and the gate insulation film orin the vicinity thereof not on purpose to maintain the insulation nor tojust increase the drain current. Based on this acquired knowledge, theinventors of the present invention further developed the research andfinally completed the present invention.

An FET according to the present invention is characterized by comprisinga gate electrode formed at one side of a base substrate, a sourceelectrode formed at the one side of the base substrate, a drainelectrode formed at the one side of the base substrate, an insulationlayer formed between the gate electrode and the source electrode andbetween the gate electrode and the drain electrode, an organicsemiconductor layer formed around (at the periphery of) the sourceelectrode and the drain electrodes, and a reformed layer attachedbetween the insulation layer and the organic semiconductor layer andcontaining a compound having the CN group in a molecule. The basesubstrate may be one that doubles as a gate electrode. Moreover, thereformed layer may be one attached between the insulation layer and theorganic semiconductor layer and composed of only a compound having theCN group in a molecule.

According to the FET having the above-mentioned configuration, it isconfirmed that a stable drain current can be obtained despite an elapseof time because the reformed layer provided so as to be interposedbetween the insulation layer and the organic semiconductor layercontains a compound having the CN group (hereinafter, referred to as thespecific compound containing the CN-group) in the molecule.

The details of the mechanism that brings about the function and effectdescribed above are not made clear yet but if the fact is taken intoconsideration that no significant effect can be obtained from astructure in which the order of lamination of the insulation layer andthe reformed layer is reversed, that is, a structure in which theinsulation layer is provided between the reformed layer and the organicsemiconductor layer, it can be thought that the function and effect isdue to the interaction between the material making up the organicsemiconductor layer and the specific compound containing the CN group atthe boundary surface between the organic semiconductor layer and thereformed layer.

More specifically, when the inventors of the present invention measured,using the method to be described later, the drain current-timecharacteristic of an FET not comprising a reformed layer, that is, anFET in which an organic semiconductor layer composed of pentacene etc.was formed directly on an insulation film composed of athermally-oxidized silicon film (SiO₂ film), it was found that the draincurrent decreased considerably as time elapsed. Moreover, it was foundthat the above-mentioned trend of the drain current in the decreasingdirection was halted by the application of a reverse bias to the gateelectrode. From these facts, it can be estimated that if the draincurrent flows, charges become more likely to be captured by the traplevel of the organic semiconductor layer and the drain current decreasesmarkedly mainly because carriers in the channel undergo Coulombscattering caused by the charges. However, the functions are not limitedto these.

In contrast to this, it can be estimated that if the specific compoundcontaining the CN group is contained in the reformed layer, charges thatcan be captured by the trap level of the organic semiconductor layer arecaused to move so as to be injected into the specific compoundcontaining the CN group from the vicinity of the boundary surfacebetween the organic semiconductor layer and the insulation layer. It canbe thought that because of the above, the extent to which the carriersin the channel undergo Coulomb scattering is reduced drastically and thedrain current is prevented from markedly decreasing as time elapses.However, the functions are not limited to these.

In the manufacture of an FET having such a structure, an organicsemiconductor layer is formed on a reformed layer by crystal growthetc., but in this case, the top surface of the reformed layer is a fieldfor new creation. Conventionally, the insulation layer is the newcreation field, therefore, there is a possibility that the state of thecrystallinity and the crystal in the vicinity of the above-mentionedboundary surface of the organic semiconductor layer may differ from theconventional state, and it can be estimated that this may contribute tothe stabilization of the drain current described above. However, thefunctions are not limited to these.

Specifically, it is preferable for the specific compound containing theCN group contained in the reformed layer to be expressed by thefollowing chemical formula 1.

[Chemical Formula 1]

In the chemical formula, R¹ represents the alkylene group or thepolymethylene group whose carbon number k is 1 to 20 and the alkylenegroup and the polymethylene group may have an ether linkage, nrepresents an integer of 1 to 2k, R², R³, and R⁴ each represent anorganic group whose carbon number is 1 to 20 independently of each otherand at least one of R², R³, and R⁴ is the alkoxy group whose carbonnumber is 1 to 5 or the alkylamino group having an alkyl chain whosecarbon number is 1 to 20, and M represents at least one kind of atom ofSi, Ti, and Al. When M is Si or Ti, m=1 and when M is Al, m=0.

According to the knowledge of the inventors of the present invention, itcan be estimated that charges tend to move excessively from the organicsemiconductor layer into the reformed layer depending on the kind of thespecific compound containing the CN group and the concentration of thecompound contained in the reformed layer. Due to this, the state ofcharges in the vicinity of the channel becomes unstable and it can bethought that the drain current becomes unstable as a result.

In contrast to this, one of the specific compounds containing the CNgroup, which is expressed by the chemical formula 1, is a so-calledsilane coupling agent modified by the CN group, and it can be consideredthat the use thereof for the reformed layer properly prevents chargesfrom being injected excessively into the reformed layer and as a result,the state of charges in the vicinity of the channel can be preventedfrom becoming unstable.

In particular, it is preferable for the reformed layer to contain2-cyanoethyltriethoxy silane as the specific compound containing the CNgroup. The 2-cyanoethyltriethoxy silane is more frequently used in theindustry and more readily available in the market than other specificcompounds containing the CN group and, at the same time, by using this,it is possible to enhance the stability of the drain current to asufficient level.

Moreover, it is preferable for the concentration of the specificcompound containing the CN group contained in the reformed layer to beless than 83 mass %, or much preferably, 5 to 50 mass %.

If the concentration is equal to or greater than 83 mass %, the draincurrent tends to increase/decrease extremely as time elapses when aconstant gate voltage is applied continuously and as a result, thechange in width of the drain current becomes markedly large. This can beconsidered because the specific compound containing the CN group iscontained in the reformed layer in a state of being properly diluted andcharges are prevented from being injected excessively from the organicsemiconductor layer to the reformed layer. If the concentration isbetween 5 to 50 mass %, the variations in the drain current can besuppressed more strongly and the mobility in the organic semiconductorlayer tends to be increased significantly and sufficiently and, as aresult, the drain current can be increased sufficiently.

Moreover, it is preferable for the reformed layer to have a thickness of0.5 to 500 nm, or much preferably, 0.5 to 100 nm.

When the thickness is less than 0.5 nm, it tends to become moredifficult to form a reformed layer having a sufficiently-enhanceduniformity of thickness in the plane. However, it is preferable for thethickness of the layer to be uniform, but it dose not matter even ifthere is a small defective portion such as a pin hole in the reformedlayer.

On the other hand, under the conditions that the above-mentionedexcessive injection of charges can be caused, or more specifically, in astate in which cyanoethyl pullulan is used for an insulation layer asconventionally, the inventors of the present invention varied thethickness of the cyanoethyl pullulan variously to manufacture an organicFET having a structure in which an organic semiconductor layer made ofpentacene is attached on the cyanoethyl pullulan film. Then theinventors measured the electrostatic capacitance-gate voltagecharacteristic, which will be described later, for each obtained organicFET. Moreover, the inventors measured the change of the drain currentwith elapsed time in a state in which a constant drain voltage wasapplied.

As a result, it is confirmed that in the case of an FET in which theelectrostatic capacitance when a negative bias gate voltage is appliedto the gate electrode exceeds a value twice or greater than theelectrostatic capacitance when a positive bias gate voltage is applied,the variations in the drain current cannot be suppressed sufficientlyand the film thickness of the cyanoethyl pullulan at this time is about1,000 nm. Based on this fact, it can be supposed that if the thicknessof the reformed layer exceeds about 500 nm, it tends to become moredifficult to sufficiently suppress the gate voltage dependency of theelectrostatic capacitance of the FET and to sufficiently prevent thevariations in the drain current. It can be estimated that one of themain reasons for the above-mentioned trend is that the charges becomemore likely to be injected excessively into the reformed layer describedabove when the thickness of the reformed layer becomes excessivelygreat.

In other words, it is preferable for the electrostatic capacitance tosatisfy the relationship expressed by the following expression(mathematical expression 1).C _(max) ≦C _(min)×2

In the expression, C_(min) indicates the minimum value of theelectrostatic capacitance in the electrostatic capacitance-gate voltagecharacteristic of the FET and C_(max) indicates the maximum value of theelectrostatic capacitance in the electrostatic capacitance-gate voltagecharacteristic of the FET.

The electrostatic capacitance-gate voltage characteristic (so-called C-Vcharacteristic) in the present invention is obtained by continuouslyapplying a negative bias from 10 V to −10 V to the gate electrode andmeasuring the electrostatic capacitance between the source electrode andthe gate electrode or between the drain electrode and the gate electrodeat a measurement frequency of 1 Hz to 1 kHz. Normally, C_(min) is anelectrostatic capacitance obtained when a positive bias is applied tothe gate electrode. In the configuration of an FET when thischaracteristic is measured, the gate electrode may be arranged inopposition to the source electrode or the drain electrode, or may be notso arranged.

On the other hand, it is preferable that the curve of the rate of changeof the drain current obtained from the drain current-gate voltagecharacteristic has a local extreme value, the first derivative issubstantially positive, or the rate of change exceeds 1 when 10 secondselapse after the gate voltage is applied.

The drain current-time characteristic (so-called I-t characteristic) inthe present invention represents the change of the drain currentcontinuously or intermittently measured during the period of time fromthe start of application (t=0) until 10⁴ seconds elapse in a state inwhich a constant gate voltage is applied continuously to the gateelectrode. The curve of the rate of change of the drain currentrepresents a curve obtained by normalizing the current values in thedrain current-gate voltage characteristic thus obtained to currentvalues at t=0. The state in which the first derivative is substantiallypositive represents a state in which the rate of change of the draincurrent increases linearly or nonlinearly without decreasingsignificantly in the curve of the rate of change of the drain current.

When the curve of the rate of change of the drain current is expressedin the form of a function exhibiting such a characteristic, it isconfirmed that the absolute value of the width of variations in thedrain current with elapsed time can be suppressed to a sufficientlysmall value.

Moreover, it is preferable for the insulation layer to be one, to thesurface or the surface layer of which, the hydroxyl group has beenintroduced. When the reformed layer is fixed on the insulation layer bycarrying out the drying process etc. of a solution-applied film formedby the application of a solution, it is preferable if the hydroxyl group(—OH) has been introduced in advance to the surface or the surface layerof the insulation layer because the adhesion of the reformed layer tothe insulation layer can be further enhanced.

EFFECT OF THE INVENTION

As explained above, according to the organic FET of the presentinvention, because of the reformed layer containing the specificcompound containing the CN group, it is possible to suppress the changeof the drain current with elapsed time after the gate voltage is appliedand obtain a stable drain current for a long time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view schematically showing an important part of aconfiguration of an FET according to the present invention;

FIG. 2 is a graph showing the drain current-time characteristic of eachorganic FET obtained in an example 1 and a comparative example 1;

FIG. 3 is a graph showing the rate of change of the drain current valuesversus elapsed time, the drain current values being obtained bynormalizing the drain currents shown in FIG. 2 to the initial values(drain currents when voltage is applied);

FIG. 4 is a graph showing the drain current-time characteristic of eachorganic FET obtained in an example 2 and a comparative example 2;

FIG. 5 is a graph showing the rate of change of the drain current valuesversus elapsed time, the drain current values being obtained bynormalizing the drain currents shown in FIG. 4 to the initial values(drain currents when voltage is applied);

FIG. 6 is a graph showing the drain current-time characteristic of anorganic FET in an example 3;

FIG. 7 is a graph showing the drain current-time characteristic of theorganic FET in the example 3; and

FIG. 8 is a graph showing the drain current-time characteristic of anorganic FET in a comparative example 3.

EXPLANATION OF REFERENCE NUMERALS

-   1 organic FET (FET)-   2 substrate (base substrate, gate electrode)-   4 composite layer-   6 source electrode-   8 drain electrode-   10 organic semiconductor layer-   41 gate insulation film (insulation layer)-   43 reformed layer

BEST MODES FOR CARRYING OUT THE INVENTION

Embodiments of the present invention are explained in detail withreference to the drawings. It is assumed that the positionalrelationship among upper, lower, right, and left parts corresponds tothat shown in the drawings.

FIG. 1 is a sectional view schematically showing an important part in aconfiguration of an FET of the present invention. An organic FET(field-effect transistor) 1 comprises a substrate 2 (base substrate) onwhich a composite layer 4 composed a gate insulation film 41 (insulationlayer) made of an insulation material and a reformed layer 43 are formedin this order, and a source electrode 6 and a drain electrode 8 arefurther arranged thereon at a predetermined distance from each other,and furthermore an organic semiconductor layer 10 is formed on andbetween the electrodes 6 and 8.

The arrangement of the organic semiconductor layer 10 is not limited tothat shown schematically. For example, an organic FET having astructure, in which the organic semiconductor layer 10 is formed on thereformed layer 43 and the source electrode 6 and the drain electrode 8are formed on the organic semiconductor layer 10 at a predetermineddistance from each other, is included in the FETs of the presentinvention.

The substrate 2 shown in FIG. 1 is made of a conductive material such aspolysilicon or doped Si, and doubles as a gate electrode. However, asubstrate having the insulating properties made of a material such asglass, ceramics, or plastic can be used and in such a case, it isnecessary to provide a gate electrode separately in order to maintainthe insulation among the source electrode 6, the drain electrode 8, andthe organic semiconductor layer 10. Moreover, materials of the organicsemiconductor layer 10 are not limited in particular provided thematerials are organic substances having the semiconductor characteristicthat can realize a channel structure, and for example, a polycycliccompound (acene) composed of four/five or more ortho condensed benzenerings in a linear arrangement such as pentacene or tetracene,polyalkylthiophene, thiophene oligomer, etc., can be used.

The gate insulation film 41 is made of various materials that exhibit aproper dielectric constant and specifically, the materials include aninorganic dielectric such as SiO₂, Al₂O₃, Si₃N₄, or TiO₂, an organicpolymer such as polyimide, Mylar, polyvinylidene fluoride,polymethylmethacrylate, etc.

The reformed layer 43 is composed of a specific compound containing theCN group, to be described later, or is prepared by solidifying apolymer, which is a matrix material (base material), in a state in whichthe specific compound containing the CN group is dissolved, dispersed,or mixed therein. Polymers for the matrix material are not limited inparticular provided the specific compound containing the CN group can beeasily dissolved, dispersed, or mixed therein, as will be describedlater, the polymers can be easily dissolved in solvents as the needarises, and the solution thereof can be easily applied, including, forexample, an acrylic resin.

Preferably, acrylic resins are polymers of (meth)acrylic acid ester basemonomers, or more specifically, polyalkyl(meth)acrylate such aspolymethyl(meth)acrylate (PMMA), polyethyl(meth)acrylate, polyn-propyl(meth)acrylate, poly n-butyl(meth)acrylate,polyisobutyl(meth)acrylate, or polytertiary butyl(meth)acrylate,multifunctional (meth)acrylate polymers, modified (meth)acrylatepolymers, etc.

Resins other than the acrylic resins include, for example, copolymers of(meth)acrylic acid ester base monomer and monomer other than this, andmore specifically, polymers of acrylamide group, polymers of aromaticvinyl compounds, etc. Moreover, examples include polyethyleneterephthalate, polyethylene, polypropylene, polyvinyl chloride, variouspolyester carbonate groups, polyurethane group, epoxy resin group, etc.

The specific compound containing the CN group is not limited inparticular provided the CN group is contained in a molecule, and anexample is a silane coupling agent modified by the CN group expressed bythe following (chemical formula 1) described above.[Chemical Formula 2]

An example of such a material is a polymer containing the cyanoalkylgroup expressed by the following (chemical formula 2).

Here, (in chemical formula 1), R¹ represents the alkylene group or thepolymethylene group whose carbon number k is 1 to 20 and the alkylenegroup and the polymethylene group may have an ether linkage, nrepresents an integer of 1 to 2k, R², R³, and R⁴ each represent anorganic group whose carbon number is 1 to 20 independently of each otherand at least one of R², R³, and R⁴ is the alkoxy group whose carbonnumber is 1 to 5 or the alkylamino group having an alkyl chain whosecarbon number is 1 to 20, and M represents at least one kind of atom ofSi, Ti, and Al. When M is Si or T, m=1 and when M is Al, m=0.

In chemical formula 2, at least one of R⁵ is the cyanide organic groupsuch as a cyanoalkyl group, such as a cyanoethyl group, or acyanoalkoxyalkyl group (ether linkage may be contained or may be not),and the rest represents the hydrogen atoms. Moreover, n is an integerbetween 1 and 20.

Among those, one expressed by chemical formula 1 and the polymer(cyanoethyl pullulan) expressed by chemical formula 2 in which at leastone of R⁵ is the cyanoethyl group are preferable, and among thoseexpressed by chemical formula 1, the 2-cyanoethyltriethoxy silane ismuch preferable.

The reformed layer 43 may not contain a polymer as a matrix material butone containing the polymer can be used preferably. In this case, theconcentration of the specific compound containing the CN group ispreferably less than 50 mass %, and much preferably, 5 to 25 mass %.Moreover, although the thickness d1 of the reformed layer 43 is notlimited in particular, the thickness is preferably 0.5 to 500 nm, andmuch preferably, 0.5 to 100 nm.

The thickness d1 of the reformed layer 43 may be designed properly inrelation to the thickness d2 of the gate insulation film 41 and it ispreferable for the thickness d1 to be set so as to satisfy, for example,the relationship expressed by the following (mathematical expression 2).d 2×0.0005≦d 1≦d 2×10

It is much preferable for the thickness d1 of the reformed layer 43 tobe set so as to satisfy the relationship expressed by the following(mathematical expression 3).d 2×0.0005≦d 1≦d 2×1

An example of a procedure of manufacturing the organic FET 1 thusconfigured is explained below. First, an n-type silicon substrate (forexample, bulk resistivity: about 10 Ωcm) is prepared as the substrate 2,and the substrate 2 is thermally oxidized according to circumstances toform the gate insulation film 41 composed of a thermally-oxidized film(SiO₂ film) having a thickness of about 200 nm.

Next, a solution is prepared, which is an organic solvent in which thepolymer for the matrix material and the specific compound containing theCN group are dissolved, or the compound containing the CN group alone isdissolved. Organic solvents are not limited in particular provided thepolymer and the specific compound containing the CN group can be readilydissolved and dispersed therein. For example, the alcohol group, theether group, the ketone group, the ester group, the glycol ether group,aromatic compounds, petroleum ether, etc., and according tocircumstances, monomers of the same kind of the polymer, for example,(meth)acrylic acid ester group, aromatic vinyl compounds, etc, can beused.

Next, the reformed layer 43 is formed using the solution thus prepared.The following two kinds of method are shown as specific methods forforming the reformed layer 43.

In the first method, the solution prepared as described above is appliedonto the gate insulation film 41 on the substrate 2. As an applicationmethod of the solution, application methods of spin-coating,roll-coating, die-coating, bar-coating, dip-coating, etc., can be usedaccording to circumstances. The solution-applied film thus formed on thegate insulation film 41 is made to undergo drying under reduced pressureat temperatures between 60 and 200° C. for, for example, 10 minutes to10 hours, thereby the reformed layer 43 is obtained. At this time, theprepared solution for application may contain a polymer for a matrixmaterial.

In the second method, first the substrate 2 having the gate insulationfilm 41 is soaked in the solution prepared as described above. Thesolution-applied film thus obtained is heated at temperatures, forexample, between 70 to 200° C. for, for example, 10 minutes to 10 hoursto make the gate insulation film to react with the gate insulation film41, thereby the film is fixed on the gate insulation film 41 and thereformed layer 43 is formed. At this time, it is preferable that priorto the formation of the reformed layer 43, the surface of the substrate2 on which the gate insulation film 41 is formed be made to undergo thehot watering treatment to properly introduce the hydroxyl group to thesurface or the surface layer of the gate insulation film 41. In thisway, the reaction between the solution-applied film and the gateinsulation film 41 is promoted during the period of heating process andthe adhesion of the reformed layer 43 is improved. Next, after theheating process for forming the reformed layer 43 is completed, it ispreferable to remove the unreacted specific compound containing the CNgroup contained in or attached to the reformed layer 43 by cleaning thesurface of the reformed layer 43.

Further, the source electrode 6 and the drain electrode 8 are formed bymetal vapor deposition of Au etc. After this, the material of theorganic semiconductor layer 10 described above is attached to theperiphery of both the electrodes 6 and the electrode 8 by the vapordeposition method etc. so that the thickness is about 50 nm to form theorganic semiconductor layer 10 and thus the organic FET 1 is obtained.The channel length is set to about 20 μm and the channel width is setto, for example, about 5 mm.

In order to manufacture the organic FET 1 having the above-mentionedconfiguration in which the organic semiconductor later 10 is provided onthe reformed layer 43 and the source electrode 6 and the drain electrode8 are formed thereon, after the reformed layer 43 is formed byapplication of the solution and the heating process of thesolution-applied film, the organic semiconductor layer 10 is evaporated,and both the electrode 6 and the electrode 8 are formed thereon by metalvapor deposition.

It is preferable for the organic FET 1 having the above-mentionedconfiguration thus obtained to satisfy the relationship expressed by thefollowing (mathematical expression 1) described above.C _(max) ≦C _(min×)2

In the expression, C_(min) represents the minimum value of theelectrostatic capacitance in the electrostatic capacitance-gate voltagecharacteristic of the organic FET 1 and C_(max) represents the maximumvalue of the electrostatic capacitance in the electrostaticcapacitance-gate voltage characteristic of the organic FET 1.

On the other hand, the organic FET 1 is useful if the curve of the rateof change of the drain current obtained from the drain current-timecharacteristic has a local extreme value, the first derivative issubstantially positive, or the rate of change exceeds 1 when 10 secondselapse after the gate voltage is applied.

In the organic FET 1 thus configured, since the specific compoundcontaining the CN group is contained in the reformed layer 43, thecharges that can be captured by the trap level that appears in theorganic semiconductor layer 10 can be caused to move so as to beinjected from the vicinity of the boundary surface between the organicsemiconductor layer 10 and the gate insulation film 41 to the specificcompound containing the CN group. Due to this, Coulomb scattering thecarriers in the channel structure would undergo, if charges werecaptured by the trap level, is reduced drastically. Therefore, the draincurrent can be sufficiently prevented from markedly decreasing as timeelapses in the organic FET 1.

If the specific compound containing the CN group is such one expressedby chemical formula 1, it becomes easier to prevent the charges fromexcessively moving from the organic semiconductor layer 10 to thereformed layer 43 and the problem of the formation of a significantamount of holes in the channel from arising. Therefore, the state of thecharges in the vicinity of the channel can be kept stable and thevariations in the drain current can be prevented much efficiently thanbefore.

EXAMPLES

The present invention is explained in detail below with reference toexamples, but the present invention is not limited to these examples.

Example 1

A highly-doped n-type silicon (bulk resistivity: 10 Ωcm) substrate thatdoubles as a gate electrode, on which a thermally-oxidized film having athickness of about 400 nm is formed as a gate insulation film, isprepared and is cut into a 25 mm by 10 mm rectangle. On the other hand,a polymer mixture of PMMA and CR-S (cyanoethyl pullulan -S type)manufactured by Shin-Etsu Chemical Co., Ltd. is dissolved in a solvent(acetone: methyl ethyl ketone: acetonitrile=2:1:1) so that theconcentration of the polymer is 1 wt %, and thus a solution is obtained.This solution is spin-coated to a silicon chip at a rotation speed of1,000 rpm, and then is made to undergo drying under reduced pressure at80° C. for an hour, and thus a reformed layer is obtained. The thicknessof the obtained reformed layer is 50 nm.

Then, pentacene, which is a material of an organic semiconductor layer,is evaporated onto the reformed layer at a film forming speed of 0.1nm/sec, and thus an organic semiconductor layer having a thickness ofabout 50 nm is formed. Further, an Au film having a thickness of about100 nm is evaporated onto the organic semiconductor layer and a sourceelectrode and a drain electrode are formed, and thus an organic FET isobtained. By the way, the channel length is set to 20 μm and the channelwidth is set to 5 mm. In the present example 1, plural organic FETshaving different concentrations of CR-S contained in the reformed layer(9 mass %, 23 mass %, 50 mass %, 83 mass %, and 100 mass %; 100 mass %means that the mass % of PMMA=0, that is, the reformed layer is made ofonly CR-S) are manufactured by properly varying the amount of PMMA andCR-S dissolved in a 10% ethanol solution.

Comparative Example 1

An organic FET is obtained in the same manner as that in the example 1except in that an application solution is prepared without using CR-S,that is, a layer made of only PMMA not containing the specific compoundcontaining the CN group is formed on a gate insulation film. In otherwords, the layer on the insulation layer in the FET in the comparativeexample 1 is made of 100 mass % PMMA.

Example 2

Plural organic FETs having the reformed layer of which differs inthickness variously (10 nm, 29 nm, 98 nm (two), and 1,100 nm) aremanufactured in the same manner as that in the example 1 except in thata solvent in which acetonitrile: N,N′-dimethylformamide=1:1 is used andthat the conditions of drying under reduced pressure after the spin-coatare that the temperature is 120° C. and the time is one hour. At thistime, the concentration of CR-S contained in the reformed layer is fixedto 100 mass %. The channel length of the organic FET having a thicknessof 10 nm is set to 50 μm.

Comparative Example 2

An organic FET is manufactured in the same manner as that in the example2, by which the organic FET having the reformed layer of which has athickness of 1,100 nm is obtained, except in that a gate insulation filmis not provided, that is, a CR-S layer having a thickness of 1,100 nm isformed as an insulation layer.

Example 3

A silicon substrate chip similar to that in the example 1 is left in aboiling water for five hours. Next, each of the silicon substrate chipsis soaked in a solution of 10 mass % ethanol of 2-cyanoethyltriethoxysilane for one hour and a dehydration polymerization reaction is causedto occur by keeping it in a state of being heated to 100° C. Then, eachsilicon substrate chip is cleaned with ethanol and is made to undergodrying under reduced pressure at 80° C. for one hour after the2-cyanoethyltriethoxy silane that has not reacted is removed, and thus areformed layer is obtained. The thickness of the obtained reformed layeris 1 to 2 nm.

Comparative Example 3

An organic FET is obtained in the same manner as that in the example 3except in that an application solution is prepared without using2-cyanoethyltriethoxy silane.

<Characteristic Evaluation 1>

The drain current-time characteristic (I-t characteristic) of eachorganic FET obtained in the example 1 and in the comparative example 1is measured by the above-mentioned method. The result is graphed andshown in FIG. 2. In the figure, the values denoted in units of mass %represent the concentration of CR-S in the reformed layer. FIG. 3 is agraph showing the change of the drain current values normalized to theinitial values (drain current when voltage is applied), that is, therate of change of the drain current with respect to elapsed time.

From these results, an increase in the drain current due to the presenceof the reformed layer containing CR-S on the insulation film isrecognized. Moreover, it is found that during the period of time fromthe application of the gate voltage until about 10³ seconds elapse, therate of change of the drain current remains in the range between about−0.5 and 1.2 compared to the initial value. As shown in FIG. 3, when theconcentration of CR-S is equal to or greater than 83 mass %, a localmaximum is recognized in the graph showing the rate of change of thedrain current. However, when the concentration is 5 to 50 mass %, nolocal maximum is recognized in the graph showing the rate of change ofthe drain current and it is understood that the variation width thereofcan be reduced comparatively and it is found that the absolute value canbe increased properly. Moreover, it is also confirmed that the draincurrent is small in the organic FET in the comparative example 1, whenthe concentration of CR-S is equal to or greater than 83%, the change ofthe drain current in a short time is considerably large, and the trendin the decreasing direction after the local maximum is marked.

<Characteristic Evaluation 2>

The drain current-time characteristic (I-t characteristic) of eachorganic FET obtained in the example 2 and in the comparative example 2is measured by the above-mentioned method. The results are shown in FIG.4. The values denoted in units of nm represent the thickness of thereformed layer (thickness of the CR-S layer as an insulation layer inthe comparative example 2). FIG. 5 is a graph showing the change of thedrain current value normalized to the initial value (drain current whenvoltage is applied), that is, the rate of change of the drain currentwith respect to elapsed time.

From these results, it can be understood that in the organic FET in theexample 2, the rate of change of the drain current is reducedconsiderably compared to that in the comparative example 2. In the casewhere the thickness of the reformed layer is 1,100 nm, the rate ofchange when 100 seconds elapse after the gate voltage is applied becomesrelatively large, and if the result of the organic FET having thedifferent thickness of the reformed layer from that is taken intoconsideration, it can be estimated that in the case of the example 2,when the thickness of the reformed layer is about 500 nm, the rate ofchange (width) of the drain current with elapsed time tends to increasecomparatively.

<Characteristic Evaluation 3>

The drain current-time characteristic (I-t characteristic) of eachorganic FET obtained in the example 3 and in the comparative example 3is measured by the above-mentioned method. The measurement is conductedunder conditions that both the gate voltage and the drain voltage are−10V. FIG. 6 and FIG. 7 are graphs showing the drain current-timecharacteristic (I-t characteristic) of the organic FET in the example 3and the dynamic range of the horizontal axis differs from each other.From these results, it is confirmed that in the organic FET in theexample 3 according to the present invention, even when 12 hours elapseafter the gate voltage is applied, a small rate of change of the draincurrent of about +30% compared to the initial value occurs.

On the other hand, FIG. 8 is a graph showing the drain current-timecharacteristic (I-t characteristic) of the organic FET obtained in thecomparative example 3. The dynamic range of the horizontal axis in FIG.8 is about 15 seconds and it can be understood that the rate of changeof the drain current in an extremely short time in the organic FET inthe comparative example 3 is incommensurably greater than that shown inFIG. 6 and FIG. 7.

INDUSTRIAL APPLICABILITY

The present invention can be used for a field-effect transistor (FET)and, more particularly, for an FET comprising a semiconductor layercontaining organic substances.

1. A field-effect transistor comprising: a gate electrode formed at oneside of a base substrate; a source electrode formed at the one side ofthe base substrate; a drain electrode formed at the one side of the basesubstrate; an insulation layer formed between the gate electrode and thesource electrode and between the gate electrode and the drain electrode;an organic semiconductor layer formed around the source electrode andthe drain electrode; and a reformed layer attached between theinsulation layer and the organic semiconductor layer and containing acompound having the CN group in a molecule.
 2. A field-effect transistorcomprising: a gate electrode formed at one side of a base substrate; asource electrode formed at the one side of the base substrate; a drainelectrode formed at the one side of the base substrate; an insulationlayer formed between the gate electrode and the source electrode andbetween the gate electrode and the drain electrode; an organicsemiconductor layer formed around the source electrode and the drainelectrode; and a reformed layer attached between the insulation layerand the organic semiconductor layer and composed of only a compoundhaving the CN group in a molecule.
 3. The field-effect transistoraccording to claim 1, wherein the compound having the CN group in amolecule contained in or making up the reformed layer is expressed bythe following chemical formula: [Chemical formula 1]

(in the chemical formula 1, R¹ represents the alkylene group or thepolymethylene group whose carbon number k is 1 to 20 and the alkylenegroup and the polymethylene group may have an ether linkage, nrepresents an integer of 1 to 2k, R², R³, and R⁴ each represents anorganic group whose carbon number is 1 to 20 independently of each otherand at least one of R², R³, and R⁴ is the alkoxy group whose carbonnumber is 1 to 5 or the alkylamino group having an alkyl chain whosecarbon number is 1 to 20, and M represents at least one kind of atom ofSi, Ti, and Al, and when M is Si or T, m=1 and when M is Al, m=0.) 4.The field-effect transistor according to claim 1, wherein the compoundhaving the CN group in a molecule contained in or making up the reformedlayer is 2-cyanoethyltriethoxy silane.
 5. The field-effect transistoraccording to claim 1, wherein the concentration of the compound havingthe CN group in a molecule contained in the reformed layer is less than50 mass %.
 6. The field-effect transistor according to claim 1, whereinthe thickness of the reformed layer is 0.5 to 500 nm.
 7. Thefield-effect transistor according to claim 1, wherein C_(min)representing the minimum value of the electrostatic capacitance in theelectrostatic capacitance-gate voltage characteristic of thefield-effect transistor and C_(max) representing the maximum value ofthe electrostatic capacitance in the electrostatic capacitance-gatevoltage characteristic of the field-effect transistor satisfy thefollowing expression:C _(max) ≦C _(min)×2.
 8. The field-effect transistor according to claim1, wherein the curve of the rate of change of the drain current obtainedfrom the drain current-time characteristic has a local extreme value,the first derivative is substantially positive, or the rate of changeexceeds 1 when 10 seconds elapse after the gate voltage is applied. 9.The field-effect transistor according to claim 2, wherein the hydroxylgroup is introduced to the surface or the surface layer of theinsulation layer.
 10. The field-effect transistor according to claim 2,wherein the compound having the CN group in a molecule contained in ormaking up the reformed layer is expressed by the following chemicalformula: [Chemical formula 1]

(in the chemical formula 1, R¹ represents the alkylene group or thepolymethylene group whose carbon number k is 1 to 20 and the alkylenegroup and the polymethylene group may have an ether linkage, nrepresents an integer of 1 to 2k, R², R³, and R⁴ each represents anorganic group whose carbon number is 1 to 20 independently of each otherand at least one of R², R³, and R⁴ is the alkoxy group whose carbonnumber is 1 to 5 or the alkylamino group having an alkyl chain whosecarbon number is 1 to 20, and M represents at least one kind of atom ofSi, Ti, and Al, and when M is Si or T, m=1 and when M is Al, m=0.) 11.The field-effect transistor according to claim 2, wherein the compoundhaving the CN group in a molecule contained in or making up the reformedlayer is 2-cyanoethyltriethoxy silane.
 12. The field-effect transistoraccording to claim 2, wherein C_(min) representing the minimum value ofthe electrostatic capacitance in the electrostatic capacitance-gatevoltage characteristic of the field-effect transistor and C_(max)representing the maximum value of the electrostatic capacitance in theelectrostatic capacitance-gate voltage characteristic of thefield-effect transistor satisfy the following expression:C _(max) ≦C _(min)×2.
 13. The field-effect transistor according to claim2, wherein the curve of the rate of change of the drain current obtainedfrom the drain current-time characteristic has a local extreme value,the first derivative is substantially positive, or the rate of changeexceeds 1 when 10 seconds elapse after the gate voltage is applied.